In the manufacture of electrodes and especially graphite electrodes, for use in electrometallurgy, e.g. in an electrosmelting furnace, or as electrodes for the production of aluminum in fused-bath electrolysis cells, the electrode blank which is insufficiently coherent and selfsupporting is supported in a sintering form by a support material filling the space between the electrode blank and the wall of the form or firing chamber.
The electrode blank which may undergo a transformation during the firing operation to a soft state through a doughy consistency, must be supported during this period and until the blank develops a sufficient stiffness so as to be self-supporting.
The choice of a support material has been conditioned in the past by a variety of factors and it is known, for example, that moisture content, particle size, bulk density or bulk weight (bulk specific gravity), abrasion resistance and glow or ignition losses are among the factors which must be considered for selection of a material which can be effectively employed as a support during the sintering process.
The moisture content can be measured in percent-by-weight of the material under consideration. The bulk density or bulk weight can be determined by filling the material without packing into a standardized volume and measuring the weight thereof, the bulk density or bulk weight being generally given in terms of kilograms/liter. The particle size can be given in milimeters for the granular material.
The abrasion characteristic can be given in terms of the resistance of the granular material to deterioration in use to fines which are much smaller in particle size than the granular material originally used. The glow or ignition loss is that fraction of the material which is transformed into the gaseous state during the sintering process.
The support material which has been utilized for this purpose in the past has generally been egg coke or crushed or screened oven coke. The glow or ignition loss in the use of such coke is very high and can equal or exceed 40% by weight which the coke had before the sintering began.
Quartz sand is also used frequently as a support material and is generally found to be unsatisfactory because of its low abrasion resistance.
During handling of the quartz sand, storage, shoveling into the forms, etc., the quartz sand tends to break down to form a substantial fraction with a particle size below 3 mm which cannot be tolerated in the support material. In addition, quartz sand has a high bulk density or bulk weight which makes handling more difficult and reduces the economy of transport of the material.